<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      button {
        display: block;
      }
      .wrap {
        display: flex;
      }
      .div {
        background-color: black;
        width: 100px;
        height: 100px;
        margin-left: 20px;
      }
    </style>
  </head>
  <body>
    <button onclick="change()">点击变颜色</button>
    <div class="wrap">
      <div class="div"></div>
      <div class="div"></div>
      <div class="div"></div>
    </div>
  </body>
</html>
<script>
  var dom = document.querySelectorAll(".div");
  function change() {
    for(let i in dom){
        dom[i].style.backgroundColor="red"
    }
  }
</script>
